Page 51 - MetalForming Magazine February 2023 - Metal Forming for the Automotive Industry

Metal Matters
Global Formability Diagram: Present and Future Steels
70
60
50
Elongation (%)
30
20 10 0
Conventional Steels
AHSS Grades
TWIP
3rd Gen AHSS under development (MedMn + Others)
3rd Gen AHSS
(DH, CH, TBF, CFB, QP)
40
IF
Mild
CMn
IF-HS
BH
TRIP
DP
FB HSLA
PQS
500
CP
0 200
800
1100 1400
MS, PHS
1700 2000
Source: WorldAutoSteel
Tensile Strength (MPa)
which collectively comprise the 1st Gen advanced high-strength steels (AHSS). Additionally, the use of press- hardening steels (PHS) in body struc- tures started to become more wide- spread as automakers faced increasingly stringent safety standards.
Just a Few 2nd Gen Steels
Although low-carbon retained- austenite steels require the use of spe- cial production techniques, the 3XX- series stainless steels are fully austenitic at room temperature. These alloys have around 18-percent Cr and 8-percent Ni, and possess excellent ductility.
Twinning-induced-plasticity ( TWIP) steels have an austenitic microstructure, achieved with a composition of 12- to 30-percent Mn, up to 3-percent Si, up to 3-percent Al, and up to 1-percent C. Due to the density of the major alloying additions relative to iron, TWIP steels have a density some 5 percent lower than most other steels. These steels have elongation values approaching 50 percent—comparable to EDDS grades. However, the tensile strength of TWIP steels approaches 1000 MPa, more than triple that of EDDS. This combination of strength and ductility helps automakers consolidate parts, which saves on stamping infrastructure while improving productivity and lowering weight. Complex-shaped parts requir- ing energy-absorption capability are
among the candidates for TWIP steel application.
While high ductility characterizes 2nd Gen steels, this benefit comes with a few significant added costs, first and foremost the expensive alloying ele- ments. Nickel currently trades near $30,000/ton, and 1 ton of 304 stainless contains about 8-percent nickel. Also, the steelmaking complexity required to achieve the proper composition and microstructure—especially in the case of TWIP steels—leads to higher pro- duction costs. Finally, the type and quantity of the necessary alloying ele- ments requires extra attention to resist- ance-welding procedures.
Where We Are: 3rd Gen Steels
Retained austenite is responsible for the transformation-induced plas- ticity effect that gives TRIP steels their name. When deforming these steels, the retained austenite in the microstructure transforms to high- strength martensite in a process that delays local necking and leads to greater ductility.
3rd Gen steels all leverage the TRIP effect to provide improved ductility in cold-forming operations as com- pared to other steels at the same strength level. Several metallurgical approaches result in these steels, each with different targets for strength and ductility.
DP steels are the most common AHSS in use. Newer grades of DP steels with increased duc- tility, dubbed DH steels, can provide as much as 4-per- cent greater elonga- tion than the com- parable DP grade at the same tensile strength, along with improved edge duc- tility. Similarly, CP steels with high ductility, known as CH steels, also have
higher elongation and improved edge ductility than comparable-strength CP steels.
TRIP-assisted bainitic ferrite (TBF) and carbide-free bainite (CFB) steels represent essentially the same type of 3rd Gen steel family. One grade in this family may reach a combination of 1000-MPa tensile strength with 13-per- cent elongation, while another might have 1200-MPa tensile strength and 10-percent elongation.
The other family of commercialized 3rd Gen steels is based on the quench- ing and partitioning (QP) process, pro- viding potential property combina- tions such as 1000-MPa tensile strength with 18-percent elongation and 1200-MPa tensile strength with 13-percent elongation. As such, they offer a cold-forming alternative to PHS in some applications.
Reaching these properties requires developing a specific balance of microstructural phases using complex annealing cycles unavailable with the equipment used in the 1990s. The nec- essary equipment and associated con- trol systems run in the hundreds of millions of dollars—one reason why only a few companies globally can pro- duce these grades. Whereas the anneal- ing cycle on conventional grades is simply heat, hold and cool, AHSS grades require quench and hold capa- bilities. 3rd Gen steels add to the com-
48 MetalForming/February 2023
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